This invention relates to a method for treatment of obstructive sleep apnea and in particular to a method of treatment in which the hypoglossal nerve is stimulated in order to open the upper airway of a patient.
Sleep apnea has been known for some time as a medical syndrome in two generally recognized forms. The first is central sleep apnea, which is associated with the failure of the body to automatically generate the neuro-muscular stimulation necessary to initiate and control a respiratory cycle at the proper time. Work associated with employing electrical stimulation to treat this condition is discussed in Glenn, "Diaphragm Pacing: Present Status", Pace, V. I, pp 357-370 (July-September 1978).
The second sleep apnea syndrome is known as obstructive sleep apnea. Ordinarily, the contraction of the dilator muscles of the upper airways (nose and pharynx) allows their patency at the time of inspiration. In obstructive sleep apnea, the obstruction of the airways results in a disequilibrium between the forces which tend to their collapse (negative inspiratory transpharyngeal pressure gradient) and those which contribute to their opening (muscle contraction). The mechanisms which underlie the triggering of obstructive apnea include a reduction in the size of the superior airways, an increase in their compliance, and a reduction in the activity of the dilator muscles. The dilator muscles are intimately linked to the respiratory muscles and these muscles respond in a similar manner to a stimulation or a depression of the respiratory center. The ventilatory fluctuations observed during sleep (alternately hyper and hypo ventilation of periodic respiration) thus favors an instability of the superior airways and the occurrence of oropharyngeal obstruction. The respiratory activation of the genioglossus has been particularly noted to be ineffective during sleep. The cardiovascular consequences of apnea include disorders of cardiac rhythm (bradycardia, auriculoventricular block, ventricular extrasystoles) and hemodynamic (pulmonary and systemic hypertension). This results in a stimulatory metabolic and mechanical effect on the autonomic nervous system. The electroencephalographic awakening which precedes the easing of obstruction of the upper airways is responsible for the fragmentation of sleep. The syndrome is therefore associated with an increased morbidity (the consequence of diurnal hypersomnolence and cardiovascular complications).
A method for treatment of obstructive sleep-apnea syndrome is to generate electrical signals to stimulate those nerves which activate the patient's upper airway muscles in order to maintain upper airway patency. For example, in U.S. Pat. No. 4,830,008 to Meer, inspiratory effort is monitored and electrical signals are directed to upper airway muscles in response to the monitored inspiratory effort. Or, in U.S. Pat. No. 5,123,425 a collar contains a sensor to monitor respiratory functioning to detect an apnea episode and an electronics module which generates electrical bursts to electrodes located on the collar. The electrical bursts are transferred transcutaneously from the electrodes to the nerves innervating the upper airway muscles. Or in U.S. Pat. No. 5,174,287 issued to Kallok, sensors monitor the electrical activity associated with contractions of the diaphragm and also the pressure within the thorax and the upper airway. Whenever electrical activity of the diaphragm suggests that an inspiration cycle is in progress and the pressure sensors show an abnormal pressure differential across the airway, the presence of obstructive sleep apnea is assumed and electrical stimulation is applied to the musculature of the upper airway. Or, in U.S. Pat. No. 5,178,156 issued to Wataru et al, respiration sensing includes sensors for sensing breathing through left and right nostrils and through the mouth which identifies an apnea event and thereby triggers electrical stimulation of the genioglossus. Or, in U.S. Pat. No. 5,190,053 issued to Meer, an intra-oral, sublingual electrode is used for the electrical stimulation of the genioglossus to maintain the patency of an upper airway. Or in U.S. Pat. No. 5,215,082 issued to Kallok et at, upon sensing of the onset of an apnea event, a stimulation generator provides a signal for stimulating the muscles of the upper airway at a varying intensity such that the intensity is gradually increased during the course of the stimulation.
One problem which has not been addressed with these methods is that of how to ensure the stimulation of the correct muscular structures in the upper airway in each patient. For example, like many peripheral nerves, the hypoglossal nerve contains fascicles innervating antagonistic muscles like the genioglossus muscle which extends the tongue and alto the styloglossus muscle which retracts the tongue. In addition, the hypoglossal nerve is near other structures which should not be stimulated. Moreover, an electrode intended to stimulate the hypoglossal nerve must be placed through a minimal sized incision which makes anatomical identification difficult and, with the patient intubated, measurements of airflow are impossible. The effects of stimulating the hypoglossal nerve have been described in animal studies such as in Abd-El-Malek, A contribution to the study of the movements of the tongue in animals, sith special reference to the cat, J. Anat. 73:15-30, 1938. Therefore, it is desirable to have a means for reliably stimulating only that portion of the nerve trunk which innervates the desired muscles or to have a means for selective placement of electrodes on specific portions of the hypoglossal nerve which are effective in opening the patient's airway.
It has been known for many years that a tripolar electrode arrangement can confine a stimulus current to the nerve cuff. (See e.g. Testerman, R. L. et al., Design and evaluation of nerve stimulating electrodes, Med Res Eng 10:6-11, 1971.) Also, as shown in Sweeny, J. D. et al., A nerve cuff technique for selective excitation of peripheral nerve trunk regions, IEEE Trans Biomed Eng 37:706-715, 1990, the tripolar arrangement of electrodes is more selective than a monopolar arrangement. This selectivity can be enhanced by the use of one or more "steering" electrodes. The electrode must be in dose contact with the nerve in order for the selectivity to work. (See e.g. McNeil, D. R. et al, Selective activation using peripheral nerve electrodes, Med & Biol Eng & Comput 23:249-253, 1985.) Several self-sizing cuff designs have been proposed which can fit snugly around the nerve without damage such as those disclosed in U.S. Pat. Nos. 4,573,481; 4,602,624; and 5,095,905. However, some of these designs tend to be fragile and difficult to install. An alternative to using these specialized electrodes is to use a more universal half-cuff design such as that disclosed in U.S. Pat. No. 5,344,438.
But, even if a half-cuff electrode is used, the surgeon implanting the electrode needs to be able to place the electrode precisely at the point on the nerve where effective stimulation can be applied to open the airway.
It is therefore an object of the present invention to provide a method for precisely locating one or more stimulation electrodes to selectively stimulate muscles to open the airway of a patient.
It is alto an object of the present invention to provide a method for selectively stimulating the nerve trunk of the hypoglossal nerve to selectively stimulate muscles to open the airway of a patient.
It is also an object of the present invention to provide apparatus for making a determination of correct electrode placement.